The present invention relates to a method for transferring image information according to the preamble of the appended claim 1, a camera module according to the preamble of the appended claim 7, and a mobile station according to the preamble of the appended claim 13.
In digital cameras and video cameras, an optical image is converted into electrical form by an image sensor, typically a charge coupled device (CCD). Such an image sensor consists of several photosensitive picture elements (pixels) which are arranged advantageously in a matrix form. The number of pixels in the image sensor affects the resolution of the image to be formed. Typically, the image sensor used in cameras and video cameras consists of hundreds of thousands of pixels, for example 640×480=307 200 pixels. In a CCD sensor, light induces a charge in the pixel, which is affected e.g. by the intensity of the light as well as the time of action of light in the pixel, i.e. exposure time. Cameras are equipped with optics whereby the image is focused at the pixels of the image sensor. When a CCD sensor is used, the pixels are uncharged before taking the picture, whereby after a predetermined exposure time, each pixel has a charge which is proportional to the quantity of light directed to it and which can be measured. After the exposure, the entry of light in the CCD sensor is prevented e.g. with a mechanical shutter. The shutter function can be implemented also electrically by sufficiently quick reading of the image sensor.
In the CCD sensor, the pixels are chained by coupling them in series, and the output of the CCD sensor is coupled with the first pixel in the connection in series, whereby the image signal from the CCD sensor can be read by transferring charges from one pixel to the next, timed by a charge transfer signal. The charges can be read from the output of the CCD element, whereby the charge of the pixel coupled to the output is read first. In the same connection, the charge transfer signal induces the transfer of charges in other pixels to the next pixel, i.e. the pixel coupled to the output will receive the charge of the second pixel coupled to the same, the second pixel will receive the charge of the pixel that is third in the connection in series, respectively, etc. Each line of the image sensor can form a separate pixel chain. Each pixel chain is provided with a separate output from the first pixel in the chain, as presented above. From these outputs from the pixel chains, the charges can be transferred e.g. to a transfer register. Reading a CCD image sensor formed in this way requires transfers of charges in a way corresponding to the number of pixels in the pixel chain. Thus, measuring the charge of a single pixel is not possible except by carrying out the transfer of charges as presented above as long as the charge of the desired pixel is in the output of the image sensor. Using such an image sensor, undersampling of the image is difficult and slow because, in practice, the charges of all pixels in the pixel chain must be transferred to the output even though some of the pixels were not processed in undersampling.
The conversion of an analog signal generated by the image sensor to digital form can be conducted with an analog/digital converter. The conversion accuracy of the analog/digital conversion is typically 8 bits, whereby 256 luminous intensity levels are obtained from each pixel. Considering the capacity of human eye, this number is usually sufficient to provide the required image quality. From the analog/digital converter, this conversion result is transferred in parallel form for further processing steps, such as for storing in an image memory or on a video tape. In digital cameras and video cameras of prior art, the display device used is an analog display device, such as a LCD display device equipped with an analog connection, whereby the image is transferred as an analog signal to the display device.
In addition to the above-mentioned CCD sensors, recent development has involved so-called CMOS image sensors, whereby it is also possible to conduct the photoelectric conversion of the image. These CMOS image sensors are based on primarily two different operating principles: integrating and non-integrating image sensors.
In integrating image sensors, the current generated by the pixel is used to charge a capacitor arranged in connection with the pixel. The charge of the capacitor depends on the strength and charge time of the current induced by the pixel. Before image formation, each capacitor is uncharged, after which the current generated by the pixel starts to charge the capacitor, whereby the charge accumulated in the capacitor after the exposure is proportional to the quantity of light to which the pixel was exposed. Setting the exposure time of integrating CMOS image sensors can be handled e.g. by a mechanical shutter, whereby the control electronics can be made simpler whereby the exposure time of each image element is substantially the same, or by timing the discharging of the capacitor and measuring of the accumulated charge substantially the same for different pixels. In an integrating image sensor, a charge is also accumulated in the capacitor when the pixel is in darkness. This may distort the image signal from the pixel. To correct this, a so-called correlated double sampling (CDS) method has been developed, whereby the charge of the capacitor of the pixel is measured after charge resetting preferably before exposure, and this value is stored for each pixel. The charge of the capacitor is measured again after the exposure time, and the stored value is subtracted from this measurement value. The difference corresponds better to the real image signal proportional to the quantity of light than an image signal obtained by one measurement. After the charge measurements presented above, the measurement value is subjected to analog/digital conversion, whereby the measurement result can be stored in digital form.
In non-integrating CMOS image sensors, the current generated by each pixel is measured, which is proportional to the intensity of light to which the pixel is exposed at the time. This kind of a sensor has the advantage that each pixel can be designated separately and the current can be measured irrespective of other pixels and exposure times. This random access is easier in integrating image sensors, if a mechanical shutter is used to set the same exposure time for different pixels.
CMOS image sensors can be also divided into passive and active image sensors. Their primary difference lies in the fact that in active image sensors, the pixel is also provided with an intensifier. This reduces the spreading of the charge of capacitors in the integrating image sensor to the next capacitors at the stage of measuring the charge, which may distort the measuring results in passive image sensors.
Irrespective of the type of the image sensor, the digitised values of the pixels are transferred for further processing typically in analog form, pixel by pixel. Thus, the image field is scanned for example line by line, starting from the first pixel on the first line. The analog image signal can be sent to be displayed e.g. by an analog display device. At the stage of further processing, the analog image signal can be converted to digital form e.g. for storage in an image memory, whereby the digital value formed from the analog signal of each pixel is stored in a memory location corresponding to the pixel in question. The image signal can be subjected to e.g. filtering and noise suppression, if necessary.
In currently known camera modules comprising an image sensor and control logic, the image information can be read either in analog form, whereby the signal must be subjected to analog/digital conversion for further processing steps, or readily converted in parallel digital form. Further, the synchronisation of image information is conducted by the control logic of the camera module in a predetermined image format, whereby typically a standard quantity of information must be transferred from each image. The quantity of information for one image depends on the number of pixels in the image sensor, i.e. the resolution, and the accuracy of the analog/digital conversion of each pixel. For example, in an image sensor consisting of 480 horizontal lines and 640 vertical lines, thereby comprising 307 200 pixels, each of which is subjected to analog/digital conversion of 8 bits, the total information of one image amounts to 2 457 600 bits.
When such a camera module of prior art is connected to a portable electronic device, such as a mobile station, one problem is the greater space needed by the parallel bus solution, compared with using a serial bus for the transfer of image information. In a typical application, information of 8 bits per pixel is used in a black-and-white image and information of 24 bits per pixel in a colour image, whereby at least 8 parallel transfer lines are needed. When a separate camera module is used, the coupling cable to be connected with the parallel bus should comprise conductors for each line of the parallel bus and also a ground conductor and possibly a power supply conductor for the camera module, whereby the coupling conductor becomes considerably more expensive and stiffer to use than a coupling cable of a serial bus containing fewer conductors. Furthermore, possible capacitive coupling between signal transfer lines in the parallel bus may cause cross-talk between adjacent conductors. Cross-talk is easily increased when the length of the conductors is increased. Furthermore, parallel data transmission complicates the structure of the device to be connected to the camera module and increases the manufacturing costs.
The use of a serial bus in solutions of prior art would typically require increasing the data transfer rate at least 8 times compared with data transfer in parallel form, if the aim is to transfer the same quantity of information in the same time. This is not always possible, because fast digital signals have very sharp edges, i.e. the rise and fall times of the signal are very short, whereby they easily induce disturbances in the operation of the electronic device as well as other electronic devices. Also, signals containing rapid changes are susceptible to distortions which may affect the reliability of the data transfer.
One disadvantage with present camera modules is their inflexibility; they produce an image in a determined form at a rate determined by the camera module itself. Information produced by camera modules of prior art cannot be easily affected, whereby it may be necessary to conduct unnecessary functions in the device receiving the image signal particularly when the quantity of image information entering the receiving device exceeds the quantity that can be utilised in the receiving device, whereby transferring the unutilised image information consumes power to an unnecessary degree. Some camera modules of this kind provide the option of adjusting how often a new image is transferred from the camera module. However, the quantity of information in each image is not changed. If the receiving device cannot process all images at the set updating rate but controls the camera module to transfer images at a slower rate, the updating rate may sink to such a low level that it can be detected in the image e.g. as discontinuous movement.
In several digital cameras, an LCD display device is presently used for displaying image information. This display device is used both as a viewfinder for directing the camera to the desired photographic subject and for observing the picture taken, whereby the picture can be taken again, if necessary. Display devices of this kind are typically analog, whereby the image signal is in analog form. When used as a viewfinder, the image displayed with a display device must be updated at a sufficient rate. The frequency of updating the image is limited by the large quantity of image information to be transferred and the limited transfer rate. This results in discontinuous movement of the image to be displayed on the display device, particularly during movement of the camera or the photographic subject. Also in several video cameras, an analog LCD display device is currently used as a viewfinder, whereby the problems are similar during video recording.